The integration of renewable energy sources into existing alternating current (“AC”) and direct current (“DC”) grids is a challenge because the power output of the sources such as wind and solar is stochastic in nature, and weather dependent. A battery energy storage system is a feasible candidate for storing the variable power, and with its timescales ranging from few seconds to hours is a promising solution for the integration of distributed renewable sources and improving reliability of the grid. Battery storage technologies are, therefore, receiving significant attention and are expected to play a key role in the transformation to a low-carbon, clean energy system. Grid connected battery systems can be categorized into different levels, ranging from low voltage (kWh range) battery systems to high voltage (MWh range) battery systems. With growing energy demands, increasing the battery capacity and converter power ratings is an area of ongoing research.
In the case of energy storage applications, several battery cells are usually connected in series and parallel to form a battery pack to increase the total voltage and current of the pack. Any mismatch in a pack due to cells chemistry, component tolerances, wiring and other known asymmetries results in the voltage imbalance of series connected cells and increase in circulating currents of parallel connected cells. These asymmetries are detrimental for a battery pack resulting in decreased lifetime, poor utilization of battery capacity and poor efficiency. Additional circuits are then needed to either passively or actively manage the battery cells. Alternatively, DC-DC converters are connected to few cells and the converter outputs are connected in series and parallel as required. With a grid connected system, a two-stage approach is commonly used, where the first converter connected to a battery increases the voltage and the second converter exchange power with a grid. This two-stage approach usually requires large and bulky filters to improve the quality of the AC waveforms.
Power electronics converters are used in wide variety of applications including energy storage systems, renewable energy systems, energy conversion, and micro-grids. Because of their wide range of applications, there has been an increasing demand for converters with high efficiency, reliability, power quality and compact size.